Resonant bodies can have two or more resonance frequencies making the design of a damping device for such a body difficult if the device is to be compact as well as effective.
One example of such a body is a steel railroad wheel which when negotiating a curved rail in particular, creates a loud screeching noise due to the wheel's rim being a resonant body excited by the vibration of the wheel rim scrubbing on the rail. A railroad wheel conventionally consists of the hub which is pressed on the railroad wheel axle, the web which radiates in the form of a disk from the hub, and the rim mounted by the web's periphery and having the tread and flange for running on the rail.
Particularly when the wheel is of the type having the rim forming a separate part of the wheel and connected to the web's periphery by a ring of non-metallic elastic material having the characteristic of rubber, the rim forms a resonant body which in the axial direction of the wheel resonates when excited, with both a low or fundamental frequency, or first mode, and a second higher pitched frequency, or first harmonic, or second mode. Higher harmonics may be involved but are of such low energy as to be unobjectionable from the noise viewpoint. In other words, the railroad rim can resonate objectionably at two frequencies, a relatively low frequency and a higher frequency, so as to cause the objectionable noise.
Proposals have been made to damp such a multi-frequency resonant body, but none has been completely satisfactory. One proposal has been to use at least two elastic metal tongues mounted at their ends and respectively tuned to the different frequencies of the resonanting body, non-metallic damping material between the tongues absorbing the energy and providing effective damping of the two vibrations of different frequencies.
The above proposal is satisfactory from the viewpoint of absorbing the energy of the resonant vibrations of differing frequencies. However, this design is not satisfactory for application to a railroad wheel rim, such as by using a multiplicity of the devices in an annular array with each device fixed to the rim, because the stacked tongues involved unacceptable thickness or height. That is to say, railroad truck assembly has various parts which cannot be cleared with assurance by such a bulky device unless the rim is recessed so the device can be nested deeply within the rim. Particularly in the case of the wheel type having the rim mounted by the web via rubber, the machining of such a necessarily deep annular recess in the rim can weaken the rim to an unacceptable extent.
The object of the present invention is to improve on the proposal to use the stacked tongues, so as to obtain the multifrequency absorption advantage but at the same time to provide a more compact device particularly adapted for use as an annular array of the devices fixed to the rim of a railroad wheel and especially when the rim is rubber mounted on the wheel web.